President Obama slammed Mitt Romney on Tuesday for "writing off a big chunk of the country" after a secretly made video showed the Republican presidential nominee dismissing Obama supporters as victims who are dependent on the government. "One of the things I've learned as president is you represent the entire country," Obama said on CBS's "Late Show with David Letterman." "My expectation is if you want to be president, you've got to work for everybody, not just for some," he said to applause from the studio audience.